openflow: Change types from uint<N>_t to ovs_be<N>.

openflow: Change types from uint<N>_t to ovs_be<N>.

I've been reluctant in the past to make wholesale changes to openflow.h
because it would be a divergence from upstream that would make comparisons
and merges more difficult.  But, in practice, no one does such comparisons
and no merges happen (because OpenFlow 1.0 is not changing).  I'd still be
inclined to resist, except that in this series I'm adding actual checking
for byte order conventions (as opposed to just documentation).